Enhanced Protein Recovery from Rapeseed Press Cake via Pressurized Liquid Extraction: Effects of pH Shifting and Process Parameters

Rapeseed press cake (RPC), the protein-rich residue from edible oil production, is currently underutilized and is primarily used as animal feed. This study aimed to investigate pressurized liquid extraction (PLE) for protein recovery from RPC using response surface methodology (RSM) with precipitation yield (PY) as the response variable. Following alkaline extraction, proteins were precipitated at their isoelectric point, and solid residues were freeze-dried to obtain protein powders. Conventional extraction (CE) under magnetic stirring at room temperature was used as a reference. The results demonstrated that increasing pH from 8 to 11 significantly enhanced protein extraction efficiency for both methods. PLE exhibited superior performance, achieving higher PY compared to CE while drastically reducing extraction time from 120 min (CE) to 6 min (PLE). Optimal conditions were identified at a solid-to-liquid ratio of 0.10 g/mL, 150 °C, and 6 min, yielding a PY of 14.9%, protein recovery in extract (PRE) of 43.8%, and protein recovery in precipitated mass (PRP) of 20.0%, with a protein content (PCP) of 647.2 mg albumin eq./g. RSM analysis identified extraction temperature as the most critical parameter for PLE, highlighting its dominant role in mass transfer. Finally, amino acid (AA) analysis revealed that protein powders were rich in essential AAs, with glutamic and aspartic acids being the most abundant. Additionally, PLE-derived protein powders exhibited enhanced solubility. This study confirms PLE as a highly promising and time-efficient technique for protein recovery from RPC, supporting the potential of sustainable valorization of agro-industrial by-products and promoting a circular economy model within the food industry.